To understand the immunopathogenesis of HIV-1, we are studying the effects of virus replication and envelope proteins on T-cell and macrophage function in vitro. To assess the effect of infection on cytolytic activity, MHC-restricted cytolytic CD4+ T cell clones have been infected with MT and TCT tropic viruses. MT virus isolates infected T cell clones more efficiently than TCT viruses, but surprisingly did not interfere with cytolytic activity. In other experiments, virus infection of macrophages did not disrupt IL-12 secretion in response to bacterial antigen stimulation and did not directly induce cytokine or chemokine secretion. Infected macrophages showed a slight decrease in secretion of chemokines in response to antigens. To study the relationship between virus variation, virulence and disease progression, we have developed virus culture systems which allow primary virus replication and spontaneous variant generation. Under these conditions virus variants arise from parental MT strains that have increased envelope affinity for CD4 and enhanced cytopathicity but retain MT tropism. In contrast to the parent MT viruses, these intermediate virus variants are sensitive to neutralizing antibodies. Intermediate isolates may represent a transition stage in virus evolution during progression to AIDS.